Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment, as examples. Semiconductor devices are typically fabricated by sequentially depositing insulating or dielectric layers, conductive layers, and semiconductive layers of material over a semiconductor substrate, and patterning the various layers using lithography to form circuit components and elements thereon.
Resistors are elements that are used in semiconductor devices in many applications. A resistor is a two-terminal electronic component designed to oppose an electric current by producing a voltage drop between the two terminals of the resistor in proportion to the current. The resistance (R) of a resistor is equal to the voltage drop (V) across the resistor divided by the current (I) through the resistor, in accordance with Ohm's law (V=IR, or R=V/I). Resistors are used as part of electronic circuits and may be formed in integrated circuits and semiconductor devices. Resistors are used in applications such as electronic filters, analog-to-digital converters, memory devices, control applications, and many other types of semiconductor device applications. Rather large resistors with high resistance values are often required in radio frequency (RF), analog, and mixed signal devices, as examples.
Transistors are elements that are also used frequently in semiconductor device applications. In the past, gate dielectric materials of transistors in semiconductor devices typically comprised silicon dioxide, which has a dielectric constant or k value of about 3.9. Gate materials of transistors in the past typically comprised polysilicon. However, in some smaller and more advanced semiconductor technologies, such as in a 32 nm technology node, as an example, the use of gate dielectric materials comprising silicon oxynitride and other high k dielectric materials such as hafnium-based dielectric materials having a dielectric constant (k) of greater than about 3.9 have begun to be a trend. Gate materials that include metal layers have also begun to be used for transistors in semiconductor devices.
In some transistor applications, it is desirable to manufacture resistors from the same material layers that transistor gates elsewhere on the chip are manufactured from. However, for some transistors that have a high k gate dielectric material and that include a metal layer in the gate stack, for example, the resistance of resistors manufactured from the same gate stack as the transistors is too low for some applications, due to the presence of the metal layer, particularly in RF applications. For example, in some RF designs, high-precision high and medium ohmic resistors are required. In some technology nodes, such as 32 nm as an example, a P+ doped polysilicon resistor may have a resistance that is dominated by the low resistance of the underlying metal layer, resulting in a sheet resistance of about 150 Ohms/square, which is too low for the requirements of some RF designs.
Removing the metal layer in the resistor region results in a higher resistance value for a resistor, but requires an additional masking level and lithography process, which adds to the complexity and expense of the fabrication process.
Thus, what are needed in the art are improved methods of fabricating resistors in semiconductor devices and structures thereof.